Don't consider 'object' always truthy (#14510)
There are two reasons I'm proposing this change. First, we know that
many subclasses of 'object' can be falsy. Second, mypy sometimes
simplifies `object | Any` into just `object`. The latter was considered
always truthy, while the prior one wasn't. Now both of them are treated
consistently. An alternative fix would be to not simplify unions like
`object | Any`, but this seems a bit ad hoc.
This only has an effect when the `truthy-bool` error code is explicitly
enabled.
Fixes #14480. This doesn't just fix the regression but fixes a more
general issue.
